1. Field of the Invention
The present invention relates to an on-vehicle path guide apparatus and a path search method for searching for a recommended path between a current position and a destination at high speed so as to display the path on a map on a display unit such as liquid crystal display or CRT display.
2. Description of the Prior Art
FIG. 41 is a block diagram showing a conventional on-vehicle path guide apparatus disclosed in, for example, JP-A 2/28800. In the drawing, a processor 411 includes a microcomputer having a CPU 417, a ROM 418, a RAM 419, and an input-output interface 420, and the component parts are interconnected through a bus 421.
A traffic information receiver 412 receives, for example, road traffic information sent from a traffic information center through a transmitter mounted to a road guide sign, a signal post, or the like. A compact disc player 413 is used as a read-only external storage containing information such as road map data. A CRT display 414 displays data such as various road traffic information which are converted into display signals in the processor 411. In an operating portion 415, a passenger can input a command by an input equipment such as key switch or light-pen. A self-contained navigation type position sensor 416 includes a geomagnetic sensor, a vehicle speed sensor, and so forth.
In the conventional on-vehicle path guide apparatus shown in FIG. 41, processing is carried out to retrieve depending upon the data such as road traffic information, and a recommended path obtained as a result of processing is provided for the passenger. For this purpose, the recommended path is retrieved and extracted by sequentially executing the following three steps: the first step of determining candidate paths between a current position and a destination, the second step of retrieving a path in which a predetermined condition is met from the candidate paths, and the third step of extracting the recommended path from the paths detected as a result of retrieval in the second step depending upon the various types of traffic information and a path selecting condition. Subsequently, for example, the resultant recommended path and the various types of road traffic information are visually displayed on a display unit such as liquid crystal display or CRT display, or are provided for the passenger through voice via speaker or the like.
The conventional on-vehicle path guide apparatus has the above structure. Hence, when the path search is made, a path cost must be calculated with respect to all the candidate paths depending upon data such as width of a road defined in road map data, length of a road in each section, and the type of road (such as city road, or national road). Consequently, a considerable time is required to provide the passenger with the recommended path to the destination. As a result, there is a problem in that a long time is elapsed before the recommended path is provided for the passenger.
Further, since the recommended path is selected depending upon the result of calculation with respect to all the candidate paths, a considerable time is required to indicate the recommended path. As a result, there is another problem in that the passenger must inconveniently wait for a long time from request to acquisition of the result.
Further, it is necessary to store the result of calculation about all the candidate paths for calculation of the recommended paths, thereby requiring a considerably large memory. As a result, the large memory becomes one factor which sets the on-vehicle path guide apparatus at high prices.